Liquid crystal fourier transform imaging spectrometer

ABSTRACT

A medical system comprising a hand-held imaging device comprising optical components including a light source to illuminate an area of medical interest, a liquid crystal variable retarder to receive light from the area of medical interest, and a retardance controller to provide a driving waveform to the variable retarder that controls retardance. The device also includes an image sensor configured to receive light from the variable retarder and to convert the received light into an output voltage signal for either the camera operation or the hyperspectral imaging operation, and communication circuitry configured to communicate imaging information based on the output voltage signal to a medical diagnostic system. The hand-held imaging device is configured to switchably perform a hyperspectral imaging and a camera operation such that the operations share at least one optical component. The diagnostic device is configured to receive the imaging information and to provide diagnostic information based thereon.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/605,625, filed May 25, 2017, which is a divisional of U.S. patentapplication Ser. No. 14/527,378, filed Oct. 29, 2014, which areincorporated herein in their entirety.

This application is related to U.S. patent application Ser. No.15/605,642, filed May 25, 2017, and U.S. patent application Ser. No.14/527,347, filed Oct. 29, 2014, which are incorporated herein in theirentirety.

BACKGROUND

Hyperspectral imaging (HSI) collects and processes information fromacross the UV, visible, and infrared portions of the electromagneticspectrum. A hyperspectral imager images a band of spectral informationat each point in a scene. HSI is frequently employed to increase thedepth of information in a scene's image, and thereby increase theimage's visual contrast beyond what can be recorded with a conventionalmonochrome or color camera. This enhanced contrast can be used to detecthard to find or camouflaged objects obscured by visual noise; it canalso aid in materials identification. It can be used to assess detailedinformation about the state of a subject, such as the ripeness of apiece of fruit. Well-known applications of HSI abound for domains asdiverse as industrial and agricultural sorting, remote sensing foragriculture and defense, threat identification, and even medicine.

The advent of smartphone technology has provided powerful, mobileplatforms that a significant fraction of the world's population carrieson their person at most times. There is a trend toward increasing thenumber and types of sensors present on smartphones, and the computingpower of these phones is correspondingly increasing. Smartphones alreadyinclude multiple image sensors, but they are not currently thought of ascandidates for HSI sensors because of the prohibitive size and cost ofexisting HSI technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first view of a device containing a hyperspectral imagingsystem.

FIG. 2 shows a second view of a device containing a hyperspectralimaging system.

FIG. 3 shows a schematic of a device including a hyperspectral imagingsystem.

FIG. 4 shows a ray diagram of a hyperspectral imaging optical path.

FIG. 5 shows a graph of phase delay versus liquid crystal voltage.

FIG. 6 shows a graph of detected intensity versus phase delay.

FIG. 7 shows a graph of detected intensity at each pixel versuswavelength.

FIG. 8 shows an embodiment of a liquid crystal device consisting ofmultiple, stacked liquid crystal panels.

FIG. 9 shows a top view of an electrode panel of one embodiment of aliquid crystal device, having a pair of electrodes on each side of asingle liquid-crystal layer

FIGS. 10A and 10B show a comparison between a standard anti-parallelalignment liquid crystal cell and an embodiment of an opticallycompensated bend cell.

FIG. 11 shows a ray diagram of an optically compensated bend cell.

FIG. 12 shows a flowchart of an embodiment of a method of operating ahyperspectral imaging system.

FIG. 13 shows a flowchart of an embodiment of a method of operating ahyperspectral imaging system.

FIG. 14 shows a flowchart of an embodiment of a method of calibrating ahyperspectral imaging system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hyperspectral imaging has many promising use cases such as for sorting,remote sensing and medical applications. The cost, size, and usabilityof this technology have limited the applications. If one could include ahyperspectral imaging component or device into many common systems,hyperspectral imaging (HSI) could become much more widespread. Forexample, smartphones are readily available technology platforms for HSIcomponents. Inclusion on the smartphone could push the boundaries ofwhat is possible with HSI, as the smartphone is a general-purposeplatform carried by most individuals for which it is easy to develop newapplications.

A current state-of-the-art hyperspectral camera might work by scanning aslit or grating, or by having liquid crystal tunable filters that allowimaging at one instantaneous wavelength band. Lower-cost hyperspectralcameras might tile optical bandpass filters on the imaging sensoritself, similar to existing red-green-blue cameras but with morediscrete optical bands, trading off with a lower spatial resolution.While liquid crystal tunable filters and optical bandpass filters allowone to obtain an image in a given wavelength band immediately, they dothis at the expense of discarding all out-of-band wavelengths.

Typically, one wants to image a scene over a range of wavelengths. It istherefore desirable to record light from all the wavelengths at once,rather than only from one spectral band at a time. This can be achievedusing optically multiplexed forms of imaging spectroscopy, such asFourier transform imaging spectroscopy, that encode wavelengthinformation into a time signal that is recorded by a detector. Anyoptically multiplexed technique, such as those exemplified in thecurrent embodiments, has gains in optical throughput, a feature known asFellgett's advantage.

Systems that use gratings and two-dimensional CCDs can record light fromall wavelengths simultaneously. This may be done by scanning a slit oversomething to be imaged. Spatial information is obtained in the longdirection of the slit and the short direction is dispersed in wavelengthwith a grating so the CCD can record all of the wavelengths at once.Spatial information in the short direction of the slit is collected asthe slit is scanned in time across the object. The system involving agrating and a slit increases the costs, the complexity and the need foralignment. The embodiments here do not incur these penalties becausethey merely add a controllable liquid crystal layer to the imaging planeof the CCD or other image sensor. In addition, the use of a slit in sucha system restricts the total optical throughput at any given time to thelight that passes through the slit. By foregoing the use of slits,Fourier transform imaging spectroscopy systems gain a throughputadvantage known as the Jacquinot advantage.

The conventional way to perform Fourier transform imaging spectroscopyis to use an imaging Michelson interferometer, which is a device thatsplits an imaging path into two arms, and that varies the length of oneof the arms while recording the recombined light on an imaging detector.This kind of HSI gains from both the Fellgett and Jacquinot advantages,but it is bulky, costly, and sensitive to vibration and misalignment.The current embodiments suffer from none of these drawbacks.

The embodiments here enable a new class of HSI sensors that have thepotential to be extremely small and low-cost, and are capable of beingintegrated anywhere image sensors are currently used, including onsmartphones. The embodiments have advantages in a general-purposeplatform like a smartphone because performance parameters such aswavelength resolution, imaging speed, and spectral bands of interest canbe selected in software and are not fixed by the hardware configuration.They can be operated in conjunction with an existing image sensor andtriggered with the same camera button without disrupting the ability ofthe sensor to take normal, non-hyperspectral images. The embodimentsleverage the computing power of smartphone platforms by shifting most ofthe system complexity to the electronic/software side, therefore keepingoverall system cost low.

In addition, the use of a smartphone or other portable device havingwireless or wired communications capability allows the device tocommunicate the raw HSI data. Alternatively, the device would transmitthe analyzed or processed HSI data for use with larger devices, such asmedical equipment, or as part of a diagnostic routine.

A portable device-based HSI sensor enables current applications of HSIat the consumer level. More importantly, as HSI becomes widely deployedon a mobile platform and as mobile app developers learn to exploit thecapabilities of the new sensor, they undoubtedly will uncover novel andinteresting uses for HSI.

An example of such a device is shown in FIGS. 1 and 2. In FIG. 1, asmartphone device 10 is shown. One must note that while this particulardevice is shown and may be discussed, no limitation to this type ofdevice is intended or should be implied. Other types of devices that canemploy this component include periscopes, optical systems, telescopes,microscopes, lightfield imaging systems, and still and video cameras. InFIG. 1, the HSI component would be in the path of the aperture 12, andmay be illuminated with a light 14.

FIG. 2 shows the display side of the smartphone 10. The display sideshows the display screen 19 with an example application 18 that wouldtrigger the use of the HSI component. Without the application triggeringthe HSI component, the HSI imaging component would be inactive, allowingnormal (color or monochrome) use of the camera without HSI. The camera16 that points at the user may also be endowed with an HSI component. Aforward-facing cell-phone hyperspectral camera may be especially usefulfor users to generate HSI images for applications such as medicalimaging, etc.

The HSI component consists at least in part of a liquid crystal variableretarder placed between optical polarizers. In certain embodiments, theoptical axis of the variable retarder is nominally at 45 degrees withrespect to the polarizers. For a given wavelength of incident light thatpasses through the first polarizer, the system oscillates betweentransmitting and not transmitting the light as the optical retardationincreases. This oscillation occurs because the retarder periodicallyalters the polarization state of the light as the retardance increases,and the output polarizer functions to alternately block or pass thelight after the retarder based on its polarization state.

The intensity oscillations as a function of optical retardance,collectively called the interferogram, occur with a period that dependson the incident wavelength. Each unique incident wavelength oscillatesin intensity at a different rate, and the intensity oscillations from acombination of incident wavelengths sum together linearly. Thewavelengths can be separated by Fourier transform of the received lightas a function of optical retardance or optical phase delay, yielding anoptical spectrum. Since the liquid crystal retarder and the image sensorare located in the same or conjugate image planes, the image sensor canindependently sample and record the intensity oscillations orinterferogram at each point in an image and use this information tocalculate a hyperspectral image.

The spectral resolution Δλ at each wavelength λ and each point in thehyperspectral image is given by the formula Δλ=2πλ/Δϕ, where Δϕ is therange of optical phase delays at which the interferogram is recorded,expressed in radians. From this formula it is apparent that to resolvewavelength differences significantly smaller than the center wavelength,interferograms must be recorded with ranges of optical phase delayΔϕ>>2π. This requirement differentiates the current embodiments fromliquid crystal tunable filters that typically do not need to scan thephase delays of their constituent liquid crystal stages beyond a rangeof 2π. Furthermore, the range of phase delay is a function of wavelengthλ, liquid crystal birefringence Δn, and position-dependent effectivethickness of the image sensor β, expressed with the following equation:Δϕ=2πΔn(λ, T, V)β(x, y)/λ. Here, birefringence is a function ofwavelength, temperature T, and liquid crystal voltage V, and expressesthe birefringence between two rays normally incident to the liquidcrystal cell with ordinary and extraordinary polarization. The change inincidence angle of the chief ray with position of a given pixel on theimage sensor, and the corresponding change in optical phase delay rangeis incorporated into the position-dependent effective thickness β.

FIG. 3 shows an internal schematic of a device such as 10 that includesan HSI component. This is merely an example of one such device. Thelight enters the device through the aperture 12 and enters the optics22, which then transfer the light to the detector 24. The detector maybe a focal plane array, which is an array of elements that reside at thefocal plane of the optics, such as a charge coupled device (CCD).Whatever the operating principle of the detector, the detector convertsthe received light into a voltage signal that can be processed by theprocessor 28. The device may also include memory 26. Memory 26 may storeinstructions to operate the processor, including instructions that comein the form of applications downloaded to a smartphone, control andconfiguration information for the processor to manipulate the optics,and data generated by the HSI component as part of the optics 22.

The optics 22 may include a relay lens or other relay optical device aswill be discussed in more detail later. The optics may also include animaging lens. Typically, imaging lenses are fixed relative to the imagesensor or detector. However, as this device may consist of a hand-helddevice, the imaging lens may be movable or the device may have otheroptical techniques to allow for image stabilization that wouldcompensate for unintended hand movement. Other optical elements may beincluded in the optics or elsewhere in the system, such as a chromaticcompensation device. The optics may be under control of processor 28.

Alternatively, the processor may receive the output signal from theimage sensor and perform image stabilization on the output signals. Theoutput signals will typically represent frames of image data detected bythe sensor with each frame acquired at a particular state of the liquidcrystal variable retarder. The processor may receive at least two outputsignals from the image sensor, each representing a frame of image datadetected at the image sensor. Ideally, there is no relative motionbetween a scene or objects in a scene and the image sensor during theacquisition of output signals corresponding to a single hyperspectralimage. However, because this case of no relative motion is not alwayspractical, the processor may perform an image analysis to determine andapply registration of regions of the image or of objects in the image,compensating such motion. This compensation may eliminate or mitigatethe imaging artifacts of images captured while the system is unstable,images of moving objects, and even relative scene motion caused by thebeam walk-off phenomenon, to be described subsequently.

Another element of the HSI imager may consist of a retardance or phasecompensation layer or compensator such as 43, shown in FIG. 4. Such acompensation layer applies a static phase delay at each point in animage, enabling sampling of a different portion of the interferogramthan would normally be sampled by applying a voltage waveform to theliquid crystal device. One embodiment of such a compensation layer wouldachieve net zero retardance in combination with the liquid crystaldevice when the HSI component is inactive; this would prevent the HSIcomponent from interfering with the normal non-hyperspectral operationof the camera.

Another kind of compensation layer would consist of multiple regions ofdifferent phase delays tiled across the image sensor. An examplecompensation layer consists of two regions, one of phase delay 0 and theother of phase delay D. The liquid crystal device can generate phasedelays from 0 to D. Therefore, the region of the image corresponding tothe region of the compensation layer with phase delay 0 will produce aninterferogram with phase delay from 0 to D, whereas the other regionwill produce an interferogram with phase delay from D to 2D. If aspecial lens were used that produced a duplicate image of a scene withone image per compensation layer region, then an interferogram of thisscene with phase delay from 0 to 2D could be formed in software bycombination of the directly recorded data. This would provide ahyperspectral image with twice the spectral resolution of aninterferogram with phase delay from 0 to D yet it would be measured inhalf the time; however, the image would have half the spatial resolutionin one dimension. In general, such retardance compensation layersproffer a convenient means of trading off spatial and spectralresolution and imaging speed.

The HSI component may have many different configurations. FIG. 4 showsone example. In the embodiment shown in FIG. 4, two unpolarized,collimated, monochromatic beams of light 32 and 34 are depicted, withthe upper beam having a shorter wavelength λ₁ than the lower beam, λ₂.The optical path has a first polarizer 40 that polarizes incident light.The liquid crystal cell 42 has an alignment orientation 45 degrees withrespect to the first polarizer.

As shown in FIG. 5, varying the voltage on the liquid crystal cell 42modifies the cell's birefringence, which in turn changes the opticalphase delay of the polarization component parallel to the liquid crystalcell's alignment direction with respect to the component that isperpendicular to the liquid crystal cell's alignment direction. Theliquid crystal cell has a controller that applies a time-dependentvoltage waveform to one or more electrodes on the LC cell. This voltagewaveform is chosen to cause the optical phase delay to change at anominally constant rate over time for a given wavelength. An outputpolarizer or analyzer 44 converts the variations in polarization inducedby the optical phase delay to variations in light intensity. One or moreof the polarizers may consist of a wire grid polarizer.

The resulting time-dependent variations in intensity are picked up by apixelated detector or focal plane array such as 24, with the detectedintensity versus phase delay shown in FIG. 6 and the detected intensityversus wavelength shown in FIG. 7. The upper curve of FIG. 6 correspondsto the detected intensity variations of the shorter wavelength ray 32 inFIG. 4, while the lower curve of FIG. 6 corresponds to the detectedintensity variations of the longer wavelength ray 34. Similarly, thepeak 46 in FIG. 7 corresponds to the shorter wavelength ray 32 while thepeak 48 corresponds to the longer wavelength ray 34.

Some embodiments achieve high optical path delays between the twopolarization components while maintaining low liquid crystal drivingvoltages and/or fast liquid crystal response times. As is known in theart of Fourier transform spectroscopy, high optical path delays yieldhigh spectral resolution, and are therefore beneficial. However, a highoptical path delay generally implies a greater total thickness of liquidcrystal. To keep the driving voltages and response times low, a singlethick liquid crystal layer can be broken up into multiple layers inseries, as shown in FIG. 8. The liquid crystal response time for anindividual cell at a given voltage scales as the square of the cellthickness, so two modules with identical path delay, one a single cell,and one split into two cells, would have a switching time differing by afactor of 4. Conversely, if the switching time is held constant, the twomodules would differ in switching voltage by a factor of 4. Theembodiment of FIG. 8 consists of 4 panels. The panel stack 52 may becontrolled by a central controller 50 that manages the optical pathdelay as well as the liquid crystal response times with proper choice ofvoltage waveforms. As is known in the art, multilayer stacks of opticalcomponents such as the embodiment shown in FIG. 8 benefit from theproper choice and application of antireflection coatings at each opticalinterface.

If one layer or cell of a multilayer liquid crystal stack has anysymmetry-breaking features, these features should be alternated oropposed between layers such that the stack as a whole retains favorablesymmetry properties. Such symmetry-breaking features can be consideredto have a polarity, which refers generally to the notion of whether asymmetry-breaking feature is directed along or in opposition to a testdirection. The polarity with which electrodes are connected to a voltagesource is one such feature, wherein the notion of polarity is immediate.The liquid crystal alignment direction is another such feature, whereinthe notion of polarity can be made concrete by considering the rubbingdirection of the upper-most electrode of a liquid crystal cell as seenin a cross-sectional depiction of the cell in a plane that is parallelto the LC directors, such as FIG. 10A. If the rubbing direction is tothe right, as shown, we can say the cell has positive polarity, whereasthe mirror image cell with the rubbing direction to the left would havenegative polarity.

For a conventional antiparallel cell, all liquid crystal molecules tendto be oriented in the same direction, which corresponds to the rubbingdirection of an electrode. Such a configuration has a first-orderdependence of optical path delay on incident light angle as the incidentangle deviates from the normal. If two antiparallel cells are stackedwith opposite polarities such that their alignment directions opposeeach other, then the first order dependencies of optical path delay onincident light angle are equal and opposite and hence cancel each otherout. Therefore, by paying careful attention to the arrangement ofpolarities of symmetry-breaking features of stacks of liquid crystalcells, it is possible to maintain an incident light angle dependence ofoptical path delay of second order or higher, as well as otheradvantageous operating characteristics.

It becomes more critical to shorten the response time of the LC cell ifthe application demands taking multiple hyperspectral images insuccession, as in a hyperspectral movie. Typically, LC cells areswitched on and then passively allowed to relax. In one embodiment, theLC cell is actively switched between a configuration with a maximaloptical phase delay and a configuration with a minimal optical phasedelay. This active switching may be implemented in many ways. In oneembodiment, each electrode of the pair of electrodes that traditionallysurround the LC material has been replaced with a pair of interdigitatedelectrodes.

FIG. 9 shows one set of interdigitated electrodes 62 and 64 on panel 60that would replace one planar electrode in a traditional configuration.This panel would be replicated on the other side of the LC material. Inone embodiment, the panels that surround the LC material would include aset of electrodes that allows one to switch the electric field between aprimarily perpendicular orientation with respect to the LC substrate,and another pair to switch the electric field to a primarily parallelorientation. By changing the voltages between each set of electrodes,the LC molecules can controllably rotate between perpendicular andparallel orientations, or more generally between an orientationproviding a minimal optical phase delay and orientation providing amaximal optical phase delay. These embodiments may be referred to asactive on and active off embodiments, wherein the LC material isactively switched between states rather than switching the material toan ‘on’ state and then passively allowing it to relax. In an alternativeembodiment, the material may still be rotated ‘uncontrollably’ withapplication of a voltage waveform having a known response, as will bediscussed in more detail later.

Another aspect of response times is the selection of the LC materialitself. When choosing an LC material, one has to balance multiplefactors such as optical birefringence, dielectric anisotropy, androtational viscosity. An LC material with high optical birefringencewould result in thinner cells that achieve the same optical retardanceas a thicker cell, with a benefit in lowered response time and/ordriving voltage, partially offset by the increased rotational viscosityof such high birefringence materials.

An LC material with a high dielectric anisotropy would produce the sameresponse as a lower dielectric anisotropy material but from a lowerdrive voltage. An LC material with a lower rotational viscosity wouldhave a faster response time than a material with a higher rotationalviscosity. As will be discussed in more detail later, material-dependentproperties such as optical dispersion and temperature dependence ofrefractive index and rotational viscosity can be calibrated out of thesystem performance, but the LC material may still be selected tooptimize the system performance post-calibration. In another embodiment,an LC material and/or LC cell preparation may be used to give the LC alarger ‘pretilt’ angle because this decreases the LC switching time.

In addition to fast response times, high viewing angles increase theusefulness of the hyperspectral imaging component. While hyperspectralimaging systems currently exist, many of them have limited viewing angledue to the angle-dependent properties of the optical filters employed.As used here, ‘viewing angle’ refers to the level of invariance of theoptical phase delay for a given wavelength and a given state of theliquid crystal component with respect to deviations of the incidentlight angle from the normal.

Typically, liquid crystal displays (LCDs) are designed as switchablehalf-wave plates between crossed polarizers that can alternate betweenlight transmitting and light obscuring states. The term ‘viewing angle’as used here differs from the conventional usage as applied to typicalLCDs, which refers to the angle that a specific contrast ratio isreached between the on and off states. In the embodiments here, the LCmay function as a high-order wave plate. Because a single point in animage will be formed with a cone of light rays that has a non-zeronumerical aperture (NA), each ray of the cone travels at a differentincident angle through the LC cell. Consider the difference in opticalphase delay between the ray with the most delay and the ray with theleast delay within the cone of light rays that form a single imagepixel. As this difference in phase approaches π radians, there becomesan equal contribution from those rays of light within this cone thattransmit through the hyperspectral component and those that are obscuredby the hyperspectral component, with a corresponding loss of contrast ofthe interferogram recorded at that image pixel.

High total optical phase delay is necessary to achieve high spectralresolution; therefore, the total variation of the optical phase delayover the incident light angles of the rays corresponding to a singleimage pixel must be significantly less than π radians. For high spectralresolution imaging, either the imaging NA has to be decreased, or theviewing angle of the LC component must be increased. It is critical todevelop techniques that increase the viewing angle to image with highspectral resolution while maintaining high optical throughput.

One particular embodiment that achieves an extended viewing angle usesparallel rubbing layers, known as a pi-cell or optically compensatedbend (OCB) cell. The two alignment layers internal to the liquid crystalcell may be rubbed in parallel directions, shown as 72 in FIG. 10B,versus in anti-parallel directions, shown as 70 in FIG. 10A. This causesthe top half of the cell to act like the mirror image of the bottom halfwith respect to a mirroring plane halfway between the bottom and tophalves of the cell, incurring similar symmetry advantages as stackingtwo anti-parallel cells with opposite alignment directions as discussedpreviously. Light rays traveling at different angles through the cell72, shown in FIG. 11, see the same optical path difference to firstorder in incident angle between ordinary and extraordinarypolarizations. This first-order invariance to incident angle arisesbecause first-order deviations in optical path difference have oppositesign in the top and bottom halves of the cell and therefore cancel eachother out.

Another embodiment involves stacking two conventional anti-parallelcells, such as 70 in FIG. 10A with the LC alignment direction of onerotated 180 degrees with respect to the other, as described previouslywith reference to FIG. 8. This would perform similarly to the pi-cellexcept with the top and bottom halves housed in separate cells stackedon top of one another, thereby providing advantages of decreased drivingvoltage and/or faster response. Other embodiments may include single- ormultiple-domain, vertically-aligned (VA) LC cells and in-plane-switching(IPS) LC cells.

When light rays travel through a birefringent medium, they can undergoan effect referred to as ‘beam walk-off’ in which the wave vector andthe Poynting vector are no longer parallel. The embodiment of two layerswith opposed symmetry provides a remedy for this, because the walk-offof the first would be corrected by the walk-off of the second. Ingeneral, symmetry-preserving arrangements of LC cells in which walk-offis cancelled between two cells or two cell halves with opposite symmetryproperties could correct this walk-off IPS LC cells would not have awalk-off issue because beam walk-off is minimal when the wave vector isperpendicular or parallel to the LC director. If walk-off is notcorrected, the image may drift as a function of voltage on the LC,creating artifacts on the edges within the image once the Fouriertransform was obtained. However, it may still be possible to correctthis walk-off algorithmically, neglecting dispersive effects of theliquid crystal, by image registration techniques.

Having demonstrated different embodiments of the structure of the HSIimaging component, the discussion now turns to additional elements andthe methods of operating the HSI imager, as well as its calibration.

As mentioned above with regard to the active on and active offembodiments, one method of operating the liquid crystal is shown in FIG.12. At 80, the electrodes, more than likely the multiple pairs discussedwith reference to FIG. 9, are activated by a voltage source. The liquidcrystal material is then rotated by a first set of electrodes to a firstorientation at 82, with either a minimal or maximal optical phase delaybetween extraordinary and ordinary polarizations, with respect to animaging sequence of optical phase delays. As needed to obtain thedesired images, the liquid crystal material may then be rotated to thesecond orientation at 84, with a maximal or minimal optical phase delay,passing through an intermediate range of optical phase delays. Thechoice of which electrodes to drive, with what voltages, may be combinedwith a choice of frequencies for each driving waveform. As shown in FIG.12, the rotation can be cycled between the two orientations repeatedly.

Some liquid crystal materials experience a change in sign of thedielectric anisotropy at a certain driving frequency. Driving thematerial below this frequency causes the molecules to align parallel orperpendicular to the driving electric field, and driving above thisfrequency causes the molecules to align at 90 degrees with respect totheir alignment at the lower frequency. This feature can be used in anactive on and active off embodiment by switching the liquid crystalbetween two orientations with the choice of drive frequency.

Many characteristics of the imaging system may require calibration. Forexample, the calculated wavelength of a monochromatic source as afunction of position over the image sensor may not appear uniform,because the LC cell may have a non-uniform thickness, and there is alsoa dependence of calculated wavelength on angle of the chief ray at eachpixel position. A calibration would take this information into accountso that a processed HSI dataset of a monochromatic source, such as alaser, would show a spectral peak at the same wavelength in all imagepixels. For example, a laser source may have a wavelength of 532nanometers. Directing light from the laser source toward the HSI imagesensor and then determining the peak wavelength at a certain point onthe image sensor may result in a peak being detected at 540 nanometers.The application software could be programmed to adjust for this offset.Due to the smoothly varying nature of this offset as a function ofposition, the calibration process may be performed at a few points orpixel-binned regions in the image plane and then interpolated across theentire image plane, or it could be performed individually at all pixels.

In addition, the index of refraction/dispersion of the LC, as well asits rotational viscosity and other material parameters, may vary as afunction of temperature, and there may be some hysteresis inherent inthe switching process. Therefore, the liquid crystal driving waveformshould be calibrated and optimized as a function of imaging speed andoperating temperature, etc. This calibration may be assisted by pointingthe HSI image sensor at a fluorescent light bulb or other light sourcethat has multiple known spectral lines, especially when opticaldispersion is present and multiple spectral peaks are needed to estimatethe dispersion.

Referring back to FIG. 1, it can be seen that there is a light source 14pointing away from the aperture to illuminate the subject being imaged.Referring to FIG. 3, one can also see a second light source 20 that mayilluminate the sensor or the elements internal to the HSI optical path.Finally, also shown in FIG. 3 is a thermometer 30.

In the calibration process, the light sources included as part of thedevice may be used as the calibration light source. The sources shouldhave known spectral characteristics that allow adjustment of the variousperformance characteristics of the hyperspectral sensor based upon thosecharacteristics, such that the calculated spectrum at each image pixelaccurately reflects the known spectrum of the calibrated source with thehighest possible spectral resolution.

FIG. 13 shows an embodiment of a calibration method. At 90, the HSIsensor is illuminated with a light source. The light source is sampled92 with the sensor. The light source may be a monochromatic inwardfacing light source, producing light rays that pass directly to thehyperspectral component rather than first reflecting off externalscenery. A performance characteristic of the sensor is calibrated 94.This may allow for calibration of the LC retardance as a function ofvoltage, or determination of an optimal LC driving waveform, forexample. A monochromatic outward facing source may be useful as aspectroscopic source, such as for imaging Raman spectroscopy. The lightsource may consist of broadband outward or inward facing sources usedfor illumination, such as infrared LEDs for night vision, or differentLEDs with specific spectral output such as a true “white light” sourcewith flatter spectral output than typical white LEDs.

In another embodiment related to calibration and switching speed, the LCmaterial does not need to be switched adiabatically between differentretardances. Rather, one can take into account the liquid crystaldynamics to create a dynamic driving waveform that monotonically changesthe optical retardance as a prescribed function of time. With such adynamic driving waveform, the transition between the desired retardancestates occurs faster than the liquid crystal response time. The opticalphase delay between the ordinary and extraordinary rays discussed abovecan be made to follow a prescribed monotonic function of time withappropriate calibration of the driving waveform. An embodiment of such amethod is shown in FIG. 14. At 100, light with known spectralproperties, such as from a monochromatic source, is received at the LCcell retarding device. The LC cell is driven with a pre-computed voltagewaveform at 102.

The pre-computed voltage waveform may come from modeling the LC materialto approximate the voltage driving waveform that would produce thedesired optical retardance versus time. This approximate voltagewaveform is then used to drive the LC while a hyperspectral image 104 iscaptured of the received light 100. Given that the spectral propertiesof the incident light are known, it is possible to calculate thetime-dependent optical retardance of the LC cell and compare it to thedesired retardance that was used to synthesize the pre-computed voltagewaveform. The voltage waveform can be adjusted based on the discrepancybetween desired and actual retardance to achieve a more accurate resultat 106. For example, the measured center wavelength can be made to moreclosely approximate a known center wavelength of received light at 100,with better spectral resolution as well, after proper adjustment of thedriving voltage waveform. Another option would be to have the retardanceversus time characteristic follow a linear trajectory. Any or all of theperformance characteristics, calibration information, and properties ofthe light used for calibration can be stored in look-up tables to allowfor comparison and adjustments.

One should note that the system may use calibration data from othersources, rather than generating the calibration data itself. While theabove discussion stores the calibration data generated by aself-calibration process, the look-up tables may also store calibrationdata provided with the system, available from other sources, etc. Thereis no limitation intended nor should any be assumed that the onlycalibration data available is if the system performs the above process.

In some embodiments it may be advantageous to perform pixel binning,whereby the intensity values recorded at neighboring pixels in a regionare summed together either directly on the image sensor or later insoftware. If the pixels are binned together at the image sensor beforethey are read out, it is generally possible to increase the frame rateof the image sensor. This assumes a fixed maximum communication speedbetween the image sensor and the device that records the data from theimage sensor. Varying the number of pixels binned thus allows one totrade-off between spatial resolution and imaging speed, and since thereis also a tradeoff between imaging speed and spectral resolution, pixelbinning is yet another method to trade off between spectral and spatialresolution and imaging speed. In addition, pixel binning may beimportant for increasing the signal to noise ratio of an image,especially when signals are weak and minimal spatial resolution isneeded. In an extreme limit, all pixels could be binned together, andthe embodiment would function as a non-imaging Fourier spectrometer.

Pixel binning may be particularly useful for calibration of varioussmoothly-varying position-dependent quantities of the HSI sensor such asthe position-dependent variation in optical phase delay at a givenwavelength and state of the liquid crystal. In order to get reliablecalibration information it may be necessary to achieve a high signal tonoise ratio, whereas not much spatial resolution would be needed becauseof the smoothly varying nature of the quantities needing calibration.Calibration information for the whole image sensor could be interpolatedfrom the measured results.

Other modifications and embodiments are possible. In order to decreaseimaging time, for example, the system could include an optical bandpassfilter, such as a Bayer filter, in front of the HSI component torestrict light hitting the sensor to a known spectral band, allowingsubsampling of the interferogram without aliasing. An electronic digitalor analog bandpass filter that filtered the signals recorded at eachimage pixel would achieve the same effect. An optical filter may alsoincrease the spectral resolution if one is imaging a spectral featurethat occurs near a sharp cutoff of a filter that has sharp cutoffs. Asused here, a sharp cutoff, or transition between the passband and thestopband, is one that is sharper than the full width at half maximum ofa spectral peak that would be obtained from a monochromatic source ifthe filter were not present. Such a filter would yield information as towhether the spectral peak occurred below or above the filter cutoff.

Other types of optical filters or optical components, such as differenttypes of optical films, may also be employed in certain embodiments. Forexample, a retardance compensation device such as a film may be used toachieve a net-zero retardance in the “on” or “off” state of the LC. Thefilter or film may also provide chromatic compensation to correct forthe dispersive effects of the liquid crystal. Or, standardantireflection coatings may be used on/in the LC cells.

The LC electrodes may consist of graphene or other material with highconductivity and high optical transparency. Especially when consideringthe multilayer embodiments of the present invention, the light reflectedor absorbed from the electrodes must be minimized.

Many of the embodiments discussed above have assumed the presence of theHSI imaging component in the same system as a traditional camera, withthe HSI component having a zero-retardance mode to allow the traditionalcamera to operate without obtrusion. However, it may be desirable tohave the HSI component not in the final focal plane of the image sensor,but instead in a conjugate of the focal plane and linked to the finalfocal plane by one or more sets of relay optics. This would enable anadd-on module to be used with existing cell phones or cameras, wherebythe add-on would be a hyperspectral component that would control or becontrolled by the existing cell phone/camera to synchronize the drivingof the liquid crystal cells with the acquisition of individual imageframes.

The methods and device above may be employed by one of severalapplications, such as medical imaging, sorting, spectroscopy of materialdiscovered in the field, etc. Each of these may have its own softwareprogram, typically referred to as an ‘app’ in the smartphone world. Asmentioned previously, an HSI component may be integrated into manydifferent types of mobile devices, such as phones, tablets, etc., aswell as more traditional lab equipment like microscopes and telescopes.

One specific application that may be enabled by this system is theability of a user to use the HSI system as a medical diagnostics device,for example for colorimetric readout of home medical diagnostic tests.The user could take one or more HSI images of body locations and/ormedical diagnostic testing strips. The resultant HSI images can be fullyor partially processed into diagnostic information using the system'sincluded processing power, or the images can be processed in the cloud.With the included communications link, the diagnostic information canultimately be forwarded to a doctor or laboratory.

It will be appreciated that several of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations, or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

What is claimed is:
 1. A medical system, comprising: a hand-held imagingdevice, comprising: optical components including: a light sourceconfigured to illuminate an area of medical interest; a liquid crystalvariable retarder configured to receive light from the area of medicalinterest; a retardance controller configured to provide a drivingwaveform to the liquid crystal variable retarder that controlsretardance of the liquid crystal variable retarder; an image sensorconfigured to receive light from the liquid crystal variable retarderand to convert the received light into an output voltage signal foreither the camera operation or the hyperspectral imaging operation; andcommunication circuitry configured to communicate imaging informationbased on the output voltage signal to a medical diagnostic system, thehand-held imaging device configured to switchably perform ahyperspectral imaging operation and a camera operation such that thecamera operation and the hyperspectral imaging operation share at leastone optical component; and the diagnostic device configured to receivethe imaging information and to provide diagnostic information based onthe imaging information.
 2. The system of claim 1, wherein one or moreof wavelength resolution, imaging speed, and spectral bands of interestfor the hyperspectral imaging operation are software selectable.
 3. Thesystem of claim 1, wherein the hand held imaging device comprises aninward facing calibration light source arranged to provide calibrationlight to components of the imaging device.
 4. The system of claim 1,wherein the area of interest is a portion of a patient's body.
 5. Thesystem of claim 1, wherein the area of medical interest is acolorimetric test strip.
 6. The system of claim 1, wherein the imagingdevice is configured to provide for calibration of the liquid crystalvariable retarder as a function of the driving waveform.
 7. A method,comprising: selecting operation of a hand-held imaging device, theselecting comprising selecting between a camera operation or ahyperspectral imaging operation, the camera operation and thehyperspectral imaging operation sharing at least one optical componentof the hand-held imaging device; providing light to an area of medicalinterest; receiving light from the area of medical interest in a liquidcrystal variable retarder of the hand-held imaging device; providing,from the hand-held imaging device, a driving waveform to the liquidcrystal variable retarder that controls retardance of the liquid crystalvariable retarder; receiving light from the area of medical interest atan image sensor of the hand-held imaging device; generating an outputvoltage signal from the image sensor responsive to the light receivedfrom the area of medical interest; communicating image information basedon the output voltage signal from the hand-held imaging device to adiagnostic system; and providing diagnostic information based on theimage information.